Studies in animals support the human observation in that steatotic livers are more sensitive to liver damage following ischemia and reperfusion (I/R) than lean livers. Basal differences between lean and steatotic livers include increased hepatocyte fat content, alterations in hepatocyte substrate metabolism, ATP content, and up-regulation of mitochondrial uncoupling protein-2 (UCP2). Further, we have shown that toll-like receptor 4 (TLR4) expression is increased in steatotic livers. We believe these differences place steatotic livers in a compromised state that leads to liver dysfunction when an additional stress, such as I/R, is encountered. We have shown that 1) mice with steatotic livers have increased liver damage including hepatocyte necrosis and apoptosis compared to mice with lean livers following I/R;2) mice with steatotic livers are more susceptible to endotoxin-induced hepatocyte injury, which increases during I/R;3) neutralization of endotoxin partially blocks hepatocyte injury following I/R in mice with steatotic livers;4) UCP2 and TLR4 are upregulated in mice with steatotic livers;5) UCP2 knockout mice with steatotic livers exhibit less hepatocyte injury following I/R;and 6) mice with steatotic livers exhibit mitochondrial dysfunction and decreased ATP levels basally and following I/R. These studies have led us to the hypothesis that: the combination of UCP2 and TLR4 upregulation sensitizes steatotic hepatocytes to cell death following I/R. To gain a clear understanding of the significance of this in steatotic livers, we have developed the following three aims: 1) Define the role(s) of UCP2 in hepatocyte necrosis and apoptosis, and mitochondrial dysfunction in mice with steatotic livers subjected to I/R. 2) Elucidate the role of the endotoxin receptor, TLR4, in the increased sensitivity of steatotic livers to I/R. 3) Using isolated hepatocytes, determine the mechanism(s) by which UCP2 increases the sensitivity of steatotic hepatocytes to necrotic cell death and mitochondrial dysfunction following hypoxia/reoxygenation (H/R) injury.